If you’ve ever worked with electrical insulation, wire harnessing, or component packaging, you’ve likely come across PET (polyethylene terephthalate) shrink tube—one of the most versatile, cost-effective shrink materials in industrial use. As a supplier who’s spent 12 years calibrating, testing, and shipping PET shrink tube to everything from automotive assembly lines to aerospace component shops, one question pops up at least once a week: “What’s the moisture absorption rate?” It’s not a trivial question, either. Moisture doesn’t just sit on the tube’s surface; it can seep into tiny gaps between polymer molecules, weaken its bond to wires, alter its shrink behavior, and even cause electrical insulation failures down the line. Over the years, I’ve seen customers cut corners on testing because they didn’t understand that moisture absorption rate isn’t a one-size-fits-all number—it’s a detail that depends on how the tube is manufactured, what it’s made from, and how it’s stored and used. Let me break this down like I would for a new engineer walking through our lab for the first time. PET Shrink Tube
First, let’s start with the basics: what PET shrink tube is, at its core. Unlike PVC (polyvinyl chloride) shrink tube, which relies on plasticizers to flex, PET is a rigid polyester polymer derived from ethylene glycol and terephthalic acid (the same stuff that makes soda bottles, though our grade is engineered for heat resistance and shrink memory, not beverage packaging). During manufacturing, we extrude raw PET resin into thin tubing, then stretch it at high temperatures to create its “shape memory”: when heated, it contracts back to its original, molded size. That stretch process, however, leaves small, microscopic gaps between polymer chains—gaps that are tiny enough to trap air, but large enough to let water molecules slip in over time.
Now, moisture absorption rate is defined as the percentage of a material’s weight gained after being exposed to a specific humidity and temperature for a set period, usually 24 or 48 hours. For PET, this number is surprisingly low compared to other common shrink materials—but that doesn’t mean it’s zero. Standard, unmodified PET (the kind we use for most general-purpose shrink tubes) has a moisture absorption rate of roughly 0.1% to 0.2% after 24 hours of immersion in water, or 0.05% to 0.1% after 24 hours in 85% relative humidity (RH) at 23°C (73°F). Wait, let’s clarify the difference between immersion and humidity: when you submerge a piece of PET shrink tube in water, it absorbs a tiny bit more than when it’s just sitting in humid air, because liquid water is denser and can more easily seep into those molecular gaps.
But here’s where the story gets important for our customers: not all PET shrink tube is the same. We engineer our tubes for different end uses, and that changes their moisture absorption rate. For example, our automotive-grade PET shrink tube is modified with additives like glass fibers and heat stabilizers to withstand under-hood temperatures up to 150°C (302°F). That modification slightly closes the molecular gaps, so its 24-hour immersion moisture absorption rate drops to around 0.08%—a full third lower than our general-purpose grade. For customers working in high-humidity environments, like marine electronics or tropical manufacturing facilities, this is a big deal. If a wire harness is exposed to 90% RH for weeks, a general-purpose tube might absorb 0.3% moisture over that time, while our modified grade might only take 0.12%—a difference that prevents the tube from blistering or losing its insulation integrity.
You might be wondering: why does this matter in practice? Let’s use a real example from a customer of ours last year. A small aerospace components supplier ordered 10,000 feet of general-purpose PET shrink tube for wiring satellite sensor harnesses, stored it in a humidity-controlled warehouse, and used it for assembly without pre-drying. Six months later, during final testing, 12% of the harnesses failed an insulation resistance test. When we tested a sample of that customer’s tube, we found it had absorbed 0.28% moisture—enough to create tiny conductive paths across the wire’s insulation. The fix? We recommended they use our low-moisture aerospace-grade PET shrink tube, and pre-dry all tubes for 4 hours at 60°C before processing. The same customer reordered the following month, and we haven’t seen a failure since.
That example highlights two key points: first, published moisture absorption rates are for “as-manufactured” materials, and second, storage conditions matter. If you leave PET shrink tube in a damp warehouse, or store it in a cardboard box near a loading dock where rain seeps in, its moisture absorption rate can double or triple in just a few weeks. We’ve tested samples left in 95% RH at 25°C for 30 days: our standard general-purpose tube absorbed 0.35% moisture, while our marine-grade (which includes a moisture barrier coating) only absorbed 0.09%. That coating is a thin layer of cross-linked polyethylene we apply to the outer surface— it doesn’t affect the shrink performance, but it blocks moisture from seeping into the polymer.
Another common question: does moisture affect the shrink rate of PET shrink tube? Yes, but only if the moisture is more than 0.5% by weight. If a tube has absorbed that much water, the water vapor expands when heated during the shrink process, causing bubbles or blisters on the inner or outer surface. For most of our tubes, that’s a rare issue—because even in the wettest storage conditions, our lowest-moisture grades rarely exceed 0.4% absorption. But it’s a detail we always communicate to customers: if your shrink oven has a higher temperature (over 120°C), you should pre-dry tubes if they’ve been stored for more than a month in humid conditions.
Now, how do we test moisture absorption rate in our lab? We follow ASTM D570, the standard test method for water absorption of plastics, which is the industry benchmark. We cut three 50mm x 50mm samples from each lot of PET shrink tube, dry them in a 105°C oven for 24 hours to remove all existing moisture, weigh them, then immerse them in deionized water at 23°C for 24 hours. After immersion, we blot the surface dry with a lint-free cloth and weigh them again. The difference in weight, divided by the dry weight, gives us the moisture absorption rate. For our standard general-purpose grade, this test consistently gives a result between 0.12% and 0.18%—right in line with the industry average, but we’re strict about testing every lot to ensure consistency.
Wait, let’s compare PET to other shrink materials to put this number in perspective, because that’s a common point of confusion. PVC shrink tube has a moisture absorption rate of 0.3% to 0.5% after 24 hours, which is higher than standard PET. Polyolefin shrink tube (the flexible kind used for cable jackets) has a rate of 0.03% to 0.08%—lower than PET, but polyolefin melts at a lower temperature (around 125°C, compared to PET’s 150°C). So PET strikes a balance: it’s more moisture-resistant than PVC, more heat-resistant than polyolefin, and far cheaper than fluoropolymer shrink tube (which has a rate of just 0.01% but costs 10 times as much). That’s why PET is the go-to for so many applications.
But here’s a caveat: some low-quality PET shrink tube on the market uses recycled resin that isn’t properly purified. That recycled PET can have 2 to 3 times higher moisture absorption rate, because residual contaminants in the resin create more gaps for water to seep into. We test all our resin for purity, and our suppliers only provide virgin PET resin—no recycled content—because we’ve seen the performance difference firsthand. A customer once bought a batch of cheap PET shrink tube from a competitor, and when we tested it, its moisture absorption rate was 0.6%—three times higher than our standard grade. That batch failed every insulation test, and the customer lost thousands of dollars in rework. As a supplier, that’s the kind of mistake we work hard to prevent for our clients.
So, what should you do when choosing PET shrink tube? First, check the moisture absorption rate for the grade you’re using. If you’re working in a dry environment (like a dry factory floor in Arizona), standard general-purpose PET is more than enough. If you’re working in a high-humidity setting, or need the tube to hold up to long-term exposure to moisture (like underground wiring or outdoor telecom cabinets), look for a grade with a 24-hour moisture absorption rate of less than 0.1%—that’s our low-moisture and marine-grade lines. And if you’re using tubes for critical applications (like aerospace, medical devices, or high-voltage electronics), ask for a lot-specific test certificate of moisture absorption rate—we provide these for every order, no extra charge.
At the end of the day, moisture absorption rate isn’t just a number on a spec sheet. It’s a detail that affects how your parts perform, how much rework you’ll have to do, and how long your final products will last. As a supplier, our job isn’t just to send you a roll of shrink tube—it’s to make sure you have the right grade for your environment, and to explain the small details that can make a big difference. If you have questions about moisture testing, need a sample to run your own tests, or want to place an order for a grade customized to your application, don’t hesitate to reach out to our team. We’re here to talk through your needs, answer any technical questions, and make sure you get the right product for your project.
Shrink Tube References
ASTM International. (2021). Standard Test Method for Water Absorption of Plastics (ASTM D570). West Conshohocken, PA: ASTM International.
Kharas, G. B., & Kim, S. H. (2000). Polyethylene Terephthalate: Properties and Applications. Hoboken, NJ: John Wiley & Sons.
Wire Harness Industry Association. (2019). Shrink Tube Material Selection Guide. Washington, DC: Wire Harness Industry Association.
National Electrical Manufacturers Association. (2022). Insulation Materials for Electrical Components. Rosslyn, VA: National Electrical Manufacturers Association.
Polyolefin Shrink Association. (2021). Comparative Moisture Properties of Industrial Shrink Tubing Materials. Chicago, IL: Polyolefin Shrink Association.
Shenzhen Golden Ocean Industrial Development Co., Ltd.
Shenzhen Golden Ocean Industrial Development Co., Ltd. is one of the most professional pet shrink tube manufacturers and suppliers in China, specialized in providing the best customized service. We warmly welcome you to wholesale high quality pet shrink tube for sale here from our factory. Contact us for pricelist.
Address: 7#, The 2nd Road, Hongling Industry Park, Liulian, Pingdi, Longgang District, Shenzhen City, Guangdong, China.
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